Fill head commit to in payload when notifying push commits for mirroring (#23215) (#23291)

Backport #23215

Just like what has been done when pushing manually:

7a5af25592/services/repository/push.go (L225-L226)

Before:

<img width="448" alt="image"
src="https://user-images.githubusercontent.com/9418365/222100123-cd4839d1-2d4d-45f7-b7a0-0cbc73162b44.png">

After:

<img width="448" alt="image"
src="https://user-images.githubusercontent.com/9418365/222100225-f3c5bb65-7ab9-41e2-8e39-9d84c23c352d.png">

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/mirror/mirror_pull.go

@@ -500,6 +500,13 @@ func SyncPullMirror(ctx context.Context, repoID int64) bool {
 			theCommits.Commits = theCommits.Commits[:setting.UI.FeedMaxCommitNum]
 		}
 
+		if newCommit, err := gitRepo.GetCommit(newCommitID); err != nil {
+			log.Error("SyncMirrors [repo: %-v]: unable to get commit %s: %v", m.Repo, newCommitID, err)
+			continue
+		} else {
+			theCommits.HeadCommit = repo_module.CommitToPushCommit(newCommit)
+		}
+
 		theCommits.CompareURL = m.Repo.ComposeCompareURL(oldCommitID, newCommitID)
 
 		notification.NotifySyncPushCommits(m.Repo.MustOwner(), m.Repo, &repo_module.PushUpdateOptions{